1. Field of the Invention
The present invention relates to a moving image coding apparatus for coding a moving image, in particular, a moving image coding apparatus for coding moving image data having different bit depths including a high bit depth.
2. Description of the Related Art
In recent years, information relating to so-called multimedia, such as an audio signal and an image signal, has been digitalized radically. In response to the digitalization of the information, compression coding/decoding techniques for image data have attracted attention. A storage capacity required for storing the image data and a frequency band required for transmitting the image data can be reduced by the compression coding/decoding techniques. Therefore, the compression coding/decoding techniques are significantly important in the multimedia industry.
The compression coding/decoding techniques compress information amount/data amount by using a high autocorrelation (that is, a high redundancy) existing between a large number of images. The redundancy of the image data includes a temporal redundancy and a two-dimensional spatial redundancy. The temporal redundancy can be reduced by using motion detection and motion compensation on a block basis. On the other hand, the spatial redundancy can be reduced by discrete cosine transform (DCT). As an image coding method using the above-mentioned techniques, there exists an H.264/MPEG-4 PART 10 (AVC) (hereinafter referred to as “H.264”).
In the field of movie production as represented by digital cinema, expectations for hyper high quality images have been raised. Therefore, studies of highly raising image quality of video images are energetically made. As one of elements for the highly raising image quality, there is an increase in a dynamic range of the video image. Specifically, a bit depth of the image data is increased from the currently used 8 bits to 10 bits or 12 bits. The highly increasing bit depth of the image data readily results in increase in the data amount of the images. Therefore, studies on an efficient coding method are now being made. For example, in Japanese Patent Application Laid-Open No. 2011-176407, input-image data is subjected to conversion processing so as to reduce a bit depth of the input image data. Then, the image data after the conversion processing is coded, while difference information between a coded image after the conversion processing and the input image having the high bit depth is also coded.
In Japanese Patent Application Laid-Open No. 2011-176407 cited above, however, the coded image data is required to be decoded so as to calculate the difference information between the coded image having a low bit depth and the input image having the high bit depth. As a result, computation cost for coding processing is disadvantageously increased. Moreover, in the H.264 described above, a conventional coding method for an 8-bit depth is directly used to deal with the highly increasing bit depth of the image data. However, the coding method for the 8-bit depth is directly used, and thus, the data amount is readily increased. Therefore, even in this case, the computation cost for the coding processing disadvantageously increases.